The present invention relates to a cathode ray tube and particularly to a cathode ray tube having an in-line electron gun structured so as to project three electron beams in a horizontal plane toward the phosphor screen.
As a picture display means in a television receiver or a monitor terminal, a cathode ray tube having a plurality of in-line electron beams, that is, a color cathode ray tube is widely used.
A cathode ray tube of this kind comprises at least an evacuated envelope including a panel portion with a phosphor screen on its inner surface and a neck portion and a funnel portion connecting the panel portion and the neck portion, a deflection device mounted in the transition region between the funnel portion and the neck portion of the evacuated envelope, and an in-line electron gun structured so as to project three electron beams in a horizontal plane toward the phosphor screen and housed in the neck portion.
FIG. 8 is a schematic view illustrating the electrode constitution of an in-line electron gun used for a cathode ray tube of this kind and FIGS. 9A and 9B are illustrations of the essential electrodes of the electron gun shown in FIG. 8. In the drawings, numeral 1 indicates a cathode, 2 a control electrode, 3 an accelerating electrode, 4 a first focus electrode, 4a an internal electrode placed in the first focus electrode 4, 5 a second focus electrode, 5a and 5b parallel electrodes for forming an electrostatic quadrupole lens, 6 a plate electrode placed in the second focus electrode 5, 7 an anode, and 8 a plate electrode placed in the anode 7.
FIG. 9A is a cross sectional view along the line 100--100 in FIG. 8, and FIG. 9B is a cross sectional view along the line 101--101 in FIG. 8, and each same numeral as that shown in FIG. 8 corresponds to the same part.
As shown in FIG. 9A, the free ends of a pair of the parallel electrodes 5a and 5b attached to the second electrode 5 on the side of the first focus electrode 4 extend into the single opening formed in the first focus electrode 4 and sandwich vertically in non-touching fashion three in-line electron beam apertures 4.sub.1, 4.sub.2, and 4.sub.3 formed in the internal electrode 4a placed in the first focus electrode 4.
In the plate electrode 6 placed in the second focus electrode 5, as shown in FIG. 9B, one elliptical aperture through which the center electron beam passes and semi-elliptical cutouts on both sides thereof are provided.
A cathode ray tube having an electron gun of the aforementioned constitution operates as follows:
Thermoelectrons emitted from the three cathodes heated by a heater are attracted toward the control electrode 2 by a positive voltage of 200 to 1000 V applied to the accelerating electrode 3 and form three electron beams.
The three electron beams pass through the apertures of the control electrode 2 and then the apertures of the accelerating electrode 3, and enters the main lens accelerated by the positive voltages applied to the first focus electrode 4, the second focus electrode 5, and the anode 7. Before the electron beams enter the main lens, a slight focusing action is exerted on them by a prefocus lens formed between the accelerating electrode 3 supplied with a low voltage of about 200 to 1000 V and the first focus electrode 4.
Furthermore, the second focus electrode 5 constituting the main lens is supplied with a low voltage of about 5 to 10 kV which is the same that of the first focus electrode 4, superposed with a dynamic voltage varying with an increase in the deflection angle of the electron beams and the anode 7 is supplied with a high voltage of about 20 to 35 kV.
An electrostatic quadrupole lens is formed on the opposing surfaces of the first focus electrode 4 and the second focus electrode 5 so as to correct for degradation of the focus characteristic at the screen corners caused by the deflection of the electron beams.
By the main lens formed by the potential difference between the second focus electrode 5 and the anode 7, the electron beams are focused on the phosphor screen and form beam spots on the screen.
The main causes for degradation of the focus characteristic which increases as the deflection angle of the electron beams increases are that firstly, since a self-converging deflection yoke is generally used to scan the electron beams on the phosphor screen, astigmatism is generated due to nonhomogeneity of its magnetic deflection field and secondly, since the distance from the main lens to the screen corners is longer than the distance from the main lens to the screen center, the electron beam focusing condition is different between the screen center and the screen corners.
Therefore, to solve the problem that the resolution deteriorates at the screen corners, an electron gun is structured to form an electrostatic quadrupole lens as shown in FIG. 9A and to receive a dynamic voltage varying with an increase in the deflection angle of the electron beams on the second focus electrode 5.
A prior art electron gun and a prior art cathode ray tube of this kind are disclosed in Japanese Patent Application Laid-open Sho 58-103752, which corresponds to U.S. Pat. No. 4,581,560, and Japanese Patent Application Laid-Open 2-72546, which corresponds to U.S. Pat. No. 4,851,741.